Method for securing access to a watch

ABSTRACT

A method for securing access to a watch ( 100 ) including: authentication ( 10 ) of the wearer of the watch ( 100 ) with a view to authorising access to the functions of this watch ( 100 ) based on an interaction between the wearer of the watch and a graphic representation included in the watch ( 100 ), and periodic checking ( 20 ) of the identity of the wearer of the watch through a verification of the validity of a digital data item determined based on at least one biometric data item of the wearer of said watch with a view to retaining/removing the authorisation for access to the functions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No.19214102.6 filed Dec. 6, 2019, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a method for securing access to a watchand a system implementing such a method.

The invention also relates to a watch comprising such a system as wellas a computer program.

PRIOR ART

A watch comprises a set of functions which can be used by the wearer.Such functions can relate in addition to setting the time or date, touse of confidential or private data specific to the wearer and usefulfor accessing customised services. These data are for example keys,certificates, authentication codes, passwords and PINs which are used toconnect the watch securely to a private corporate network,authentication with respect to secure servers such as a bank server, ora secure messaging system for sending and receiving signed and/orencrypted emails. As such, it is understood that it is important to beable to secure access to the use of the functions of such a watch.

For this purpose, in the prior art, methods are known envisagingsecuring access to a watch and in particular to the functions of thiswatch, by reinforcing the authentication criteria authorising thisaccess for example by implementing additional authentication steps.

However, one of the major drawbacks of such methods is linked with thefact that, once the wearer of the watch is authenticated, it is thenpossible for any individual to obtain access to the functions of thewatch particularly in the event of the latter being stolen.

It is understood that there is a need to find an alternative solution,particularly which does not have the drawbacks of the prior art.

SUMMARY OF THE INVENTION

An aim of the present invention is consequently that of proposing amethod for securing access to a watch which is reliable and robust.

Such a method has the advantage of ensuring the identity of the wearerof the watch automatically, transparently and non-intrusively for thewearer.

In this aim, the invention relates to a method for securing access to awatch including the following steps:

-   -   authentication of the wearer of the watch with a view to        authorising access to the functions of this watch based on an        interaction between the wearer of the watch and a graphic        representation comprised in said watch, and    -   periodic checking of the identity of the wearer of the watch        through a verification of the validity of a digital data item        determined based on at least one biometric data item of the        wearer of said watch with a view to retaining/removing the        authorisation for access to said functions.

In further embodiments:

-   -   the authentication step comprises a presentation sub-step of a        graphic representation on a visual data broadcasting interface        of said watch;    -   the authentication step comprises a selection sub-step within a        limited time interval of a sequence of at least two        identification portions comprised in said graphic representation        aiming to identify said wearer, said sequence corresponding to        an identification code of the wearer;    -   the authentication step comprises a validation sub-step of the        selected sequence;    -   the checking step comprises an acquisition sub-step by at least        one multispectral biometric skin sensor comprised in the watch,        of a plurality of images of a portion of the wearer's skin        adjacent to said sensor, said images comprising said at least        one biometric data item comprised in this skin portion;    -   the checking step comprises a sub-step of generating a digital        identification item based on said at least one biometric data        item;    -   the checking step comprises a sub-step of validating a digital        identification item generated in anticipation of the wearer        identity check;    -   the validation sub-step comprises a comparison phase including a        sub-phase of retaining the authorisation to access said        functions of the watch if the digital identification item is        substantially similar or similar to a reference digital        identification item;    -   the acquisition sub-step comprises an illumination phase of the        skin portion along different wavelengths;    -   the acquisition sub-step comprises a capture phase of images of        the skin portion illuminated at different wavelengths;    -   the generation sub-step comprises a characterisation phase of        said at least one biometric data item comprised in the acquired        images relative to said skin portion;    -   the generation sub-step comprises a design phase of the digital        identification item based on the characterisation of said        biometric data item;    -   the biometric data item relates to a vascular network or to a        texture of this skin.

The invention also relates to a system for securing access to a watchimplementing this method, the system comprising the followinginterconnected elements: a processing unit, a biometric sensor, an inputinterface and a visual data broadcasting interface.

Advantageously, the biometric sensor is a multispectral biometric skinsensor.

The invention also relates to a watch, particularly a connectedmechanical watch, including such a system.

The invention also relates to a computer program comprising program codeinstructions for executing the steps of the method when said program isexecuted by a processing unit.

BRIEF DESCRIPTION OF THE FIGURES

Further specificities and advantages will emerge clearly from thedescription given hereinafter, which is by way of indication and in noway limiting, with reference to the appended figures, wherein:

FIG. 1 is a schematic representation of a watch comprising a system forsecuring access to a watch, according to an embodiment of the invention,and

FIG. 2 is a logic diagram relating to a method for securing access to awatch, according to the embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a watch 100 comprising a system for securing access to awatch 100 and in particular to the use of functions of this watch 100.Such a system 1 is comprised in the watch 100 which is preferably ahybrid-display connected mechanical watch 100. In this context, thiselectronic device 100 comprises a body such as the watch case, and anattachment element such as a bracelet making it possible for this bodyto be fastened for example to the wearers wrist. This system 1 morespecifically comprises in a non-limiting and/or non-exhaustive manner:

-   -   a processing unit 2 including hardware and software resources in        particular at least one processor cooperating with memory        elements 6;    -   a visual data broadcasting interface 3 such as a hybrid display        dial provided with a first analogue display component and a        second digital and/or alphanumeric display component;    -   an audio data broadcasting interface 4 such as a speaker;    -   a wireless communication interface 5 (for example cellular, WLAN        Bluetooth, etc.);    -   an input interface 34 such as a keypad or indeed a        touch-sensitive interface comprised for example in the visual        data broadcasting interface 3, and    -   a biometric sensor 35 particularly in this embodiment a        multispectral skin sensor, said sensor 35 comprising at least        one photographic sensor 36, at least one multispectral light        source 37 and at least one thermal image sensor 38, the light        source 37 being capable of emitting luminous radiation in        wavelengths between 300 and 1100 nm, furthermore, this light        source 37 can be of the laser type.

In this system 1, the processing unit 2 is connected inter alia to thevisual and audio data broadcasting interfaces 3, 4, to the inputinterface 34 as well as to the wireless communication interface 5 and tothe biometric sensor 35. It will be noted additionally that thebiometric sensor 35 is arranged in the body of the electronic device 100and/or in the attachment element.

This system 1 implemented in the electronic device 100 is capable ofchecking the identity of the authenticated wearer of the watchdiscreetly without direct intervention/interaction of the wearer withthis watch 100, so that they can use the functions of the watch 100 allthe time that they are wearing it, without having to undergoauthentication again. The wearer is then identified transparently anddiscreetly, based on at least one biometric data item. Within the scopeof the present embodiment, the biometric data item is comprised in aportion of the wearer's skin and can be a vascular network of the skinor indeed a texture of this skin. This skin of the wearer covering theirbody has a specificity, less obvious for those skilled in the art toenvisage as not naturally seen by the human eye, linked with theabsorption and reflection characteristics at different wavelengths(spectrum) of the components of the skin, located at different depths.In a simplified model, the skin consists of a semi-transparent layerknown as the “epidermis” located on the surface then, below theepidermis, a layer known as the “dermis” and comprising, inter alia, theblood vessels (or vascular network) wherein haemoglobin is highlyreflective at high wavelengths near red being comprised for examplebetween 760 and 930 nm which makes it possible herein to reveal ordetect the vascular network of the wearer's skin. In other words, thelight absorption spectrum of the components of the epidermis and thedermis forming the skin not being uniform along electromagneticwavelengths, the appearance and colour of the skin are the result of acomplex combination of these phenomena. Thus, when it is necessary todetect or reveal a biometric data item such as the skin texture of thiswearer, a texture essentially formed of cracks and cavities, the skincan then be illuminated by a light source restricted to the wavelengthsaround red which tends to make the shadow phenomenon at the bottom ofcracks disappear. Indeed, a retroprojection effect occurs by reflectingthese near-red wavelengths onto the dermis and through the epidermis,whereas illuminating the skin by a source of a colorimetric spectrum ata distance from red, typically the band of wavelengths located betweenviolet (400 nm) and up to orangey yellow (600 nm), makes it possible onthe other hand to contrast these skin cracks significantly through theappearance of shadows at the bottom of these cracks. It will be notedthat the identification of a biometric data item comprised in the skincan be enhanced using the thermal image sensor 38 preferably withoutillumination. By way of example, for the detection of the skin textureparticularly when the portion of this wearer's skin concerned is endowedwith hairs, the use of the thermal image sensor 38 makes it possible toreveal the cracks of this skin texture which are generally warmer thanthe surrounding skin and the hairs colder than this surrounding skin.Thus, in this configuration, the hairs can be distinguished thermallyfrom the cracks of the skin texture due to this difference between therespective temperature thereof.

It will be noted that the thermal images can be captured underillumination in a given wavelength according to the biometric data itemto be detected or revealed.

It is therefore understood, according to the principle of the invention,that the periodic identification of the wearer is carried out based onat least one biometric data item comprised in images of a portion ofthis wearers skin which can be illuminated where applicable alongdifferent wavelengths in order to capture the images comprising thebiometric data item sought. Thus, this biometric data item, comprised inthese images, can be detected by the illumination performed in differentwavelengths or without illumination for example when capturing thermalimages is involved.

In this system 1, the memory elements 6 of the processing unit 2comprise data relating to a previously defined/generated referencedigital identification item. These memory elements 6 also includedigital image processing algorithms 39 making it possible tocharacterise at least one biometric data item relating to the wearersskin and which is comprised in the images relative to the portion of thewearers skin. These memory elements 6 also include generation algorithms40 of the reference digital identification item but also of the digitalidentification item which is generated periodically within the scope ofthe wearer's identity check.

These memory elements 6 of the processing unit 2 also comprise at leastone graphic representation 41 envisaged to take part in theauthentication process of the wearer so that the latter has access tothe functions of the watch 100. This graphic representation 41 can forexample be an image comprising at least one object. By way of example,this image defines a scene comprising a plurality of objects such ashouses, vehicles and/or a celestial body such as the moon, etc. It isobviously understood that this image can define further types of sceneincluding at least one object. These memory elements 6 also include datarelating to a reference sequence 42 comprising reference identificationportions of this graphic representation 41, said portions having beenpreviously selected by the wearer of the watch during the configurationof an authentication process.

Such a system 1 of the watch 100 is capable of implementing a method forsecuring access to the watch 100 and in particular to the use offunctions of this watch 100, represented in FIG. 2 . Such a method aimsto check the identify of an authenticated user discreetly, i.e.non-intrusively and automatically, so that they can use the functions ofthe watch 100 all the time that they are wearing it, without having toundergo authentication again. The functions of the watch 100 can relateto setting the time, date, or indeed using a timer, a navigation system,a pedometer, an activity logger, a messaging system, access toconfidential or private data specific to the wearer and useful foraccessing customised services, etc.

This method comprises an authentication step 10 of the wearer of thewatch 100 with a view to authorising access to the functions of thiswatch 100 based on an interaction between the wearer of the watch and agraphic representation comprised in said watch 100 more specifically agraphic representation displayed on/in the visual data broadcastinginterface 3 of said watch 100. This step 10 enables the wearer to provetheir identity. More specifically, this step 10 comprises a sub-step 11of presenting a graphic representation 41 on the visual databroadcasting interface 3 of said watch 100. This sub-step 11 includes adisplay generation phase 12, on/in the visual data broadcastinginterface 3, of the graphic representation 41 envisaged for theimplementation of this authentication. This phase 12 can comprise aselection sub-phase by the user from a sample of at least two graphicrepresentations 41 displayed on the visual data broadcasting interface3, of the graphic representation 41 envisaged for the implementation ofthis authentication. it will be noted that the wearer is the only one toknow the graphic representations 41 that they selected during theconfiguration of the authentication process for access to the functionsof the watch 100.

This presentation sub-step 11 then comprises a countdown activationphase 13 once the generation phase 12 is carried out. In other words,the preconfigurable countdown is activated once the graphicrepresentation 41 is presented on the broadcasting interface 3. Such aphase 13 helps from a limited time interval defined by this countdown,to count down the estimated time required for the input of the sequenceof identification portions of the graphic representation 41 displayedon/in the broadcasting interface 3.

Subsequently, the authentication step 10 comprises a selection sub-step14 within a limited time interval of a sequence of at least twoidentification portions of said graphic representation 41 aiming toidentify said wearer, said sequence corresponding to an identificationcode of the wearer. Such identification portions are not directlyvisible in the graphic representation 41 presented on/in thebroadcasting interface 3. Under these conditions, the selection sub-step14 comprises a visualisation phase 15 of at least one of saididentification portions of the sequence in said graphic representation41. This visualisation phase 15 comprises a selection sub-phase of atleast one zone of interest of the graphic representation 41 suitable forcomprising said at least one identification portion. During thissub-phase, the wearer selects for example a first zone of interest or asecond zone of interest by making an enlargement of this first zone orthis second zone using the input interface 35. Once this first or secondzone of interest has been selected, the identification portions thenbecome visible. In this configuration, each identification portionusable for creating/forming the sequence can be selected using the inputinterface 35.

It should be noted that the sequence comprises an ordered number ofidentification portions and that the zone of interest selected cancomprise for example three identification portions of which merely twoare ordered successively one after the other in the sequence. In thiscontext, the remaining identification portion requires to be part of thesequence, the selection of an identification portion comprised in afurther zone of interest of the graphic representation 41.

Then, the authentication step 10 comprises a validation sub-step 16 ofthe selected sequence. This validation sub-step 16 comprises a checkingphase 17 that the selection of the sequence of identification portionshas been carried out within the time interval defined by the countdown.Insofar as this selection has been carried out within this limited timeinterval, the validation sub-step 16 then comprises a comparison step18, implemented by the processing unit 2, between said selected sequenceand the reference sequence 42. This comparison phase 18 comprises asub-phase of prohibiting access to the functions of the watch 100 ifsaid sequence is substantially different or different from the referencesequence 42. Conversely, this comparison phase 18 comprises a sub-phaseof authorising access to the functions of the watch 100 if said sequenceis substantially similar or similar to the reference sequence 42.

Insofar as this selection has not been carried out within this limitedtime interval, the validation sub-step 16 comprises a repeating step 19of the presentation 11 and selection 14 sub-steps. If, subsequently, theselection of the sequence has once again not been carried out within thelimited time interval, the validation sub-step 11 then envisages asub-step of blocking/locking access to the functions of the watch 100.In this context, a specific unlocking/unblocking procedure is thenrequired to restore access to said watch.

This method then comprises a periodic checking step 20 of the identityof the wearer of the watch through a verification of the validity of adigital data item determined based on at least one biometric data itemof the wearer of said watch with a view to retaining/removing theauthorisation for access to said functions. Such a step is also known asperiodic discreet checking step. Within the scope of the presentembodiment, the biometric data item is comprised in a portion of thewearer's skin. During this step 20, the period associated with thechecking is regular or irregular and can be configured automatically bythe processing unit 2 or defined by the wearer. By way of example, thisperiod can be a few seconds or a few minutes. Such a checking step 20makes it possible to verify following each period that the wearer of thewatch is always the same by checking their “biometric” identity based ona digital identification item established according to at least onebiometric data item comprised in this wearer's skin.

This checking step 20 then comprises an acquisition sub-step 21 by thesensor 35 of a plurality of images of a portion of the wearers skin,said skin being arranged adjacently to said sensor, said imagescomprising at least one biometric data item comprised in this skinportion. This sub-step 21 comprises an illumination phase 22 of the skinportion along different wavelengths. More specifically, during thisphase 22, the processing unit 2 controls the multispectral biometricsensor 35 and in particular the light source 37 such that the latteremits luminous radiation towards the skin portion along a specificwavelength adapted to detecting or revealing said at least one specificbiometric information item of the skin, herein sought within the scopeof the wearer identity check, and which is comprised in this portion ofthe wearers skin. Once the illumination has been configured, theacquisition sub-step 21 comprises a capture phase 23 of images of thisskin portion illuminated at at least one wavelength suitable fordetecting or revealing said at least one biometric data item. Duringthis phase 23, the processing unit 2 controls the multispectralbiometric skin sensor 35 and in particular the photographic sensor 36synchronously with the activation/deactivation of the light source 37,in order to carry out the capture of at least one image relative to theilluminated skin portion for at least one wavelength.

This acquisition sub-step 21 can also comprise a capture phase 24 of atleast one thermal image of the skin portion. Such a phase 24 is carriedout preferably without illumination but in further alternatives anillumination of the portion can be performed in at least one givenwavelength, this being dependent obviously on the biometric data item tobe detected or revealed. This phase 24 can be carried out before orafter the illumination 22 and image capture 23 phases.

The checking step 20 then comprises a generation sub-step 25 of thedigital identification item based on said at least one biometric dataitem comprised in the acquired images of the skin portion. Such asub-step 25 comprises a characterisation phase 26 of said at least onebiometric data item comprised in the images relative to said skinportion. During this phase 26, the processing unit 2 implementsprocessing algorithms 39 of the images acquired aiming toidentify/detect in each thereof said at least one biometric data itemcomprised therein. As mentioned above, this can involve data itemsrelating for example to the texture of the skin or to the vascularnetwork comprised in this portion of the wearer's skin. Theimplementation of these algorithms 39, 40 by the processing unit 2 canfor example envisage a segmentation process of these images. It isunderstood herein that each image acquired gives an overview of theportion of the wearer's skin, and then includes variable relevance zonesfor the identification of said at least one biometric data item. Such asegmentation process helps extract the segments to be processed andremove the parts not to be processed in these images. These algorithms39 can then envisage an indexing of these image segments comprisingcharacteristics relating to said at least one specific biometric dataitem to be identified, by location zones in the skin portion, in orderto be able to assign to each zone the suitable processing with respectto the morphological typology of the characteristic of this geographiczone of the portion. In this context, these algorithms 39 process eachsegment of these images by detecting the data carried by the pixels ofeach of these images by carrying out processing, transformation anddetection type image analysis operations. Subsequently, these algorithms39 perform characteristic filtering and extraction or vectorisationoperations, in order to convert the image data relating to said at leastone identified and extracted biometric data item, into parametric data,typically relative numerical values expressed for example as an index orpercentage.

It is understood herein that the acquisition of several imagesrepresenting the same skin portion helps enhance the precision andefficacy of this characterisation phase 26.

Subsequently, the generation sub-step 25 comprises a design phase 27 ofthe digital identification item based on the characterisation of said atleast one biometric data item. During this phase 27, the processing unit2 implements generation algorithms 40 of such a digital identificationitem specifically envisaged for processing the parametric data obtainedduring the characterisation phase 26, said parametric data relating tothe biometric data item.

Then, the checking step 20 comprises a validation sub-step 20 of thedigital identification item generated in anticipation of the weareridentity check. This validation sub-step 28 comprises a comparison phase29, implemented by the processing unit 2, between the digitalidentification item generated and the reference digital identificationitem. In this method, the reference digital identification item can becreated, once the wearer has been duly authenticated and their identityis assured, during a definition step 11 of this reference digitalidentification item envisaging sub-steps similar to the acquisition 21and generation 25 sub-steps implemented during the checking step 20. Inthis method, once the wearer of the watch 100 is authenticated, theprocessing unit 2 implements this definition step 11 and then archivesthe reference digital identification item obtained in the memoryelements 6 of the processing unit 2. This reference digitalidentification item can therefore be determined automatically by theprocessing unit 2 or configured by the wearer during a setting processintended to guide the wearer in the definition of this reference digitalidentification element.

This comparison phase 29 comprises an access authorisation removalsub-phase 30 to said functions of the watch 100 if the digitalidentification item generated is substantially different or differentfrom the reference digital identification item. In this scenario, theaccess to the functions of the watch 100 is then removed as the wearerand owner of the watch 100 may no longer be in possession thereof. Assuch in this context to use the functions of the watch 100 again, it isnecessary to undergo authentication.

The comparison phase 29 also comprises an access authorisation retentionsub-phase 31 to said functions of the watch 100 if the digitalidentification item generated is substantially similar or similar to thereference digital identification item. In this case, the checking unit 2once again executes, according to the determined period, the sub-stepsof acquisition 21 and generation 25 of the checking step 20 in order tocarry out the comparison phase 29 once again.

Thus, the invention makes it possible to continuously automatically andnon-intrusively check the identity of the wearer of the watchparticularly once they are authenticated.

The invention also relates to a computer program comprising program codeinstructions for executing steps 10 to 31 of this method when saidprogram is executed by the processing unit 2 of the watch 100.

The invention claimed is:
 1. A method for securing access to a watch(100) including the following steps: authentication (10) of the wearerof the watch (100) with a view to authorising access to the functions ofthis watch (100) based on an interaction between the wearer of the watchand a graphic representation comprised in said watch (100), and periodicchecking (20) of the identity of the wearer of the watch through averification of the validity of at least one digital data itemdetermined based on at least one biometric data item of the wearer ofsaid watch with a view to retaining/removing the authorisation foraccess to said functions, wherein the at least one digital data itemcomprises a feature of a surface of a skin of the wearer and the featureis obtained from, of the skin, detecting hair and cracks of the surfaceat a skin portion at which the wearer wears the watch, wherein theperiodic checking (20) further comprises verification of the validity ofones of the plurality of digital data items, comprising the cracks inthe skin texture and the hair, by thermal imaging and detecting, fromthe thermal imaging, the cracks as warmer portions of the skin thanother portions of the skin, wherein the periodic checking (20) furthercomprises an acquisition sub-step (21) by at least one multispectralbiometric skin sensor (35) comprised in the watch (100), of a pluralityof images of the skin portion of the skin adjacent to said sensor (35),said images comprising the at least one digital data item comprised inthe skin portion, and wherein the multispectral biometric skin sensor(35) is configured to detect the cracks by contrasting a first image, ofthe images of the skin portion obtained by illuminating the skin portionat one or more of first wavelengths, with a second image, of the imagesof the skin portion obtained by illuminating the skin portion at one ormore second wavelengths different than the first wavelengths, anddetecting the cracks based on detecting shadows, apparent in one of thefirst image and the second image but not the other of the first imageand the second image, by said contrasting the first image with thesecond image.
 2. The method according to claim 1, further characterisedin that the authentication step (10) further comprises a presentationsub-step (11) of a graphic representation (41) on a visual databroadcasting interface (3) of said watch (100).
 3. The method accordingto claim 1, further characterised in that, the authentication step (10)further comprises a selection sub-step (14) within a limited timeinterval of a sequence of at least two identification portions of saidgraphic representation (41) aiming to identify said wearer, saidsequence corresponding to an identification code of the wearer.
 4. Themethod according to claim 3, further characterised in that theauthentication step (10) further comprises a validation sub-step (16) ofthe selected sequence.
 5. The method according to claim 1, furthercharacterised in that the checking step (20) further comprises ageneration sub-step (25) of a digital identification item based on theat least one digital data item.
 6. The method according to claim 5,further characterised in that the checking step (20) further comprises avalidation sub-step (28) of the digital identification item generated inanticipation of the wearer identity check.
 7. The method according toclaim 6, further characterised in that the validation sub-step (28)comprises a comparison phase (29) including an access authorisationretention sub-phase (30) to said functions of the watch if the digitalidentification item is substantially similar or similar to a referencedigital identification item.
 8. The method according to claim 7, furthercharacterised in that the acquisition sub-step (21) further comprises anillumination phase (22) of the skin portion along different wavelengths,the different wavelengths comprising the first wavelengths and thesecond wavelengths.
 9. The method according to claim 8, furthercharacterised in that the acquisition sub-step (21) further comprises acapture phase (23) of images of the skin portion illuminated atdifferent wavelengths.
 10. The method according to claim 9, furthercharacterised in that a generation sub-step (25) comprises acharacterisation phase (26) of the at least one digital data itemcomprised in the acquired images relative to the skin portion.
 11. Themethod according to claim 10, further characterised in that thegeneration sub-step (25) further comprises a design sub-step (27) of thedigital identification item based on the characterisation of the atleast one digital data item.
 12. The method according to claim 11,further characterised in that the at least one digital data item relatesto a texture of the skin at the skin portion.
 13. A system for securingaccess to a watch (100) implementing the method according to claim 1,the system (1) comprising the following interconnected elements: aprocessing unit (2), a biometric sensor (35), an input interface (34)and a visual data broadcasting interface (3).
 14. The system accordingto claim 13, further characterised in that the biometric sensor (34) isthe multispectral biometric skin sensor.
 15. A watch (100), particularlya connected mechanical watch (100), including a system (1) according toclaim
 13. 16. A non-transitory computer readable medium comprisingprogram code instructions for executing the steps of the methodaccording to claim 1 when said program is executed by a processing unit(2).
 17. The method according to claim 1, wherein the periodic checking(20) further comprises verification of the validity of a plurality ofdigital data items, including the at least one digital data item, andwherein the plurality of digital data items comprise, from the skinportion at which the wearer wears the watch, the cracks in the skintexture at the surface of the skin and the hair from the surface of theskin.